Traditionally, digital oscilloscopes capture, store and subsequently display representations of electronic signals. In order to acquire and store the data, the data must be acquired by an acquisition system. Typically, this acquisition is performed by defining a trigger position at a particular time, triggering at the particular time, sampling and digitizing an analog waveform at a predetermined time relative to the trigger, and storing the sampled, digitized data for a predetermined period of time after the trigger. In a sampling oscilloscope, the data input signal is typically at a frequency too fast for the acquisition system to acquire enough points sequentially to accurately represent the waveform. Therefore, as is shown in FIG. 1, in such an oscilloscope, if a repetitive waveform signal 110 is presented, one or more samples 150 are taken during each pass of the waveform. Thus, at the start of each waveform 110, one or more triggers 120 are defined, and data is sampled at a predefined delay (n, n+1) relative to each trigger, and stored, Thus, as is shown, on subsequently provided waveforms, a sampling point 130a, 130b, . . . progresses along the waveform. A sample 150 is taken at each sample point. By varying the delay between the trigger and the various sample points, over a number of the repetitive waveforms, data representing all portions of the waveform may be acquired and assembled, therefore presenting a single composite representation of the waveform.
However, in order to perform this type of acquisition, the waveform must be repetitive and stable. It must have a regular period and start at a substantially consistent time interval. If not, it is not possible to present a known delay to the sampling points after the start of the trigger, and therefore it would not be possible to insure that all desired data will be acquired.
A coherent timebase offers another method for acquiring samples from a repetitive waveform. A system for implementation of such a coherent timebase is shown in FIG. 2. A coherent timebase consists of a phase locked loop that synchronizes to an external trigger or clock and produces strobes that are applied to the sampler at the front end of the oscilloscope. The frequency of the phase locked loop does not precisely match that of the input signal (i.e. there is an offset), and therefore over time, various portions of the waveform are sampled. Eventually, after an amount of time in which all corresponding portions of the waveform are sampled, the locations of the samples in the waveform begin to repeat. The samples of the input waveform are taken at times corresponding to the strobe pulses, and the samples are then displayed on the oscilloscope screen based on a particular algorithm that puts the samples in the correct time order. Thus, the algorithm is aware when the sample corresponding to a particular strobe fits along the waveform, and is able to reconstruct the waveform by placing all of the samples in the correct timing relationship for the waveform.
In such a coherent timebase a trigger or clock must be available that is synchronous to the signal. This trigger or clock could be the signal itself, some clock derived from the signal, or a clock synchronously related to the signal. In FIG. 2, a clock signal 215 from a clock source 210 is used as a reference input to a Phase Locked Loop (PLL) 220. The PLL is programmed (via programming software commands 265 issued from a programming software 260) to provide an output strobe 225 that has a known frequency, which as noted above has a predetermined relationship to the frequency of the repetitive input signal. The output strobe drives a sampler 230 of the sampling scope. Samples 235 are acquired from a signal 245 generated from a signal source 240 successively at the strobe rate of the PLL, and therefore at predetermined locations along various waveforms of the repetitive waveform. After the required numbers of samples are acquired, they are processed by an algorithm that puts them in time order relative to the repetitive waveform. They are then displayed on a display 250 of an oscilloscope.
However, when utilizing a coherent timebase scheme to acquire samples from a waveform, a stable waveform may not be displayed on the screen because the strobe frequency generated by the PLL is offset from the frequency of the clock input. Successive acquisitions therefore have an arbitrary phase with respect to the previous acquisition so data points making up the waveform have a relative arbitrary time position on the screen. This may manifest itself in a waveform that appears to change position along the time axis on the display each time the display data is updated.
Therefore, it would be beneficial to be able to provide an improved acquisition and display system that works with a coherent timebase.